Multibeam antennas are commonly used in spatial communications, on board a satellite (transmission of telemetry data, telecommunications), or on the ground (satcom terminal or user terminal of a telecommunications system). Among multibeam antennas, continuous linear radiating aperture antennas using a parallel-plate waveguide beamformer allow a plurality of beams to be formed over a wide angular sector. They moreover operate in a very wide band, because of the absence of resonant propagation modes. It is thus possible to obtain a multibeam continuous linear radiating aperture antenna that operates simultaneously at 20 and 30 GHz. They are lastly capable of radiating over a very vast angular sector, and have a much higher performance than an array of a plurality of radiating elements.
It is known to use a lens-like quasi-optical beamformer that will achieve collimation of the beams. The sources of the lens-like quasi-optical beamformer generate cylindrical waves, and the beamformer allows them to be converted into plane waves. FIGS. 1A and 1B illustrate such a quasi-optical beamformer. A parallel-plate waveguide 20 allows the waves to be guided in transverse-electromagnetic (TEM) mode, in which the electric field E and the magnetic field H vary in directions perpendicular to the propagation direction. The wave fronts are curved in the XZ-plane; in order to compensate for this curvature of the wave front, at least one lens, which may be of straight profile or of curvilinear profile, and which introduces a delay that is continuously variable along the X-direction, is provided. Straight-profile lenses comprise a protrusion 13 and an insert 17. These lenses are said to be straight-profile lenses because the protrusion and the insert have a straight and rectilinear profile in the XZ-plane. The height of the protrusion (along the Y-axis) is larger at the centre than at the sides and therefore a larger delay is created at the centre 14 of the protrusion than at the lateral edges 15, 16, the dimensions of the protrusion 13 being such that a plane wave front thus exits from the beamformer. A straight-profile lens allows the waves issued from a single central source 10 placed at the focal point of the lens to be correctly converted.
In contrast, when, in order to generate a plurality of beams, a plurality of sources 10 are distributed, with a distribution of curvilinear profile, around a central source 10c, a straight-profile lens may induce defocus aberrations due to the distance of the sources 10 with respect to the focal point. To solve this problem, it is possible to use what is called a curvilinear-profile lens, the profile of which is for example parabolic or elliptical. This type of lens is said to be a curvilinear-profile lens because the protrusion 13 and the insert 17, in addition to having a height that varies along the Y-axis (larger in the centre than at the sides), have a profile that is curvilinear in the XZ-plane, as illustrated in FIGS. 1C and 1D. Curvilinear-profile lenses, because of their geometry, are capable of correctly converting the cylindrical wave fronts emitted by a plurality of sources 10 that are also distributed curvilinearly in the XZ-plane. Use of curvilinear-profile lenses allows a larger number of focal points to be employed, and therefore a better beam quality to be obtained over a given angular sector. The degrees of freedom allowing a beamformer to be endowed with a plurality of focal points are in particular the curvilinear distribution of the sources 101, 102, . . . , 10M, and the input and output curvatures of the protrusion, which correspond to the internal and external curvatures of the lens, respectively. The use of what are called curvilinear-profile lenses that have an input and output curvature that is variable in the XZ-plane thus advantageously adds an additional degree of freedom with respect to a straight-profile lens. Thus, the beams emitted by off-centred sources are better formed than with a straight-profile lens.
FIGS. 2A and 2B illustrate the operating principle of a pillbox beamformer, used in a CTS antenna of the prior art, which is described below. The incident cylindrical waves, emitted by at least one source 10, are emitted into a lower parallel-plate waveguide 21, then are reflected using a reflector, called a pillbox junction 23, towards an upper waveguide 22. The pillbox junction 23 is curved, and for example of parabolic or elliptical shape. It will be noted that the pillbox junction is a type of straight-profile lens, and the pillbox-junction quasi-optical beamformer is equivalent to a straight-profile-lens quasi-optical beamformer. Specifically, the straight-profile lens and the pillbox junction have the same curvature because they must introduce the same delay to convert a cylindrical wave into a plane wave. The only difference that there may be is that the beamformer may have a straight bend before and/or after the straight-profile lens that it contains whereas a pillbox beamformer comprises no bend other than the variable-height one of the junction.
Those skilled in the art may find, in patent application EP 3 113 286 A1, more details on quasi-optical beamformers comprising straight-profile lenses and/or curvilinear-profile lenses.
A radiating aperture, for example a horn, then allows the waves made plane by the beamformer to be radiated. However, a horn coupled to a parallel-plate waveguide necessarily has a shape that is very elongated along the X-axis, and therefore produces beams that are highly elliptical along the Y-axis. Thus, the beams have different widths, in particular in the main E- and H-planes of radiation, this being unsatisfactory. One way known to those skilled in the art of obtaining identical beamwidths in the two E- and H-planes therefore consists in arraying longitudinal horns, thereby dividing the parallel-plate waveguide issued from the beamformer into a plurality of sub-guides. The signals issued from the beamformer are thus divided using a distributor, for example based on one or more parallel-plate “T” dividers, then radiated via a plurality of juxtaposed horns, thus generating a circular beam, which is much better suited to satellite communications. The distributor is thus used to divide the power at equal amplitude and phase for the various horns.
The arrangement of a distributor at the output of a pillbox-type quasi-optical beamformer is known as a continuous transverse stub (CTS) antenna. The document “Continuous Transverse Stub Array for Ka-Band Applications” (Ettore et al., IEEE Transactions on antennas and propagation, vol. 63, no. 11, November 2015) describes such an antenna. FIG. 3A shows a perspective view of a CTS antenna, and FIG. 3B a cross section cut in the YZ-plane. The CTS antenna consists of a source 10, which may be an input feed, of a parallel-plate waveguide 20, of a pillbox junction 23, of a distributor 1, and of longitudinal radiating horns 5. When the source 10 is placed at the centre of the parallel-plate waveguide 20, along the X-axis, the width (dimension along the X-axis) of the longitudinal radiating horns 5 and of the distributor 1 is generally equal to that of the pillbox beamformer along the same axis. This is because the waves emitted by the central source are not or not greatly reflected from the edges of the distributor 1, and thus few reflections occur from the edges of the distributor 1.
FIG. 4 schematically illustrates, via an exploded view, the CTS antenna described in the document “Continuous Transverse Stub Array for Ka-Band Applications” (Ettore et al., IEEE Transactions on antennas and propagation, vol. 63, no. 11, November 2015), and equipped with a plurality of sources 101, 102, . . . , 10M. The use of a plurality of sources 10 allows as many separate and simultaneous signals to be generated, which signals propagate in different but coplanar directions, in the XY-plane in the interior of the parallel-plate waveguide 20, then in the XZ-plane in the distributor 1 and after emission via the longitudinal radiating horns 5. When the antenna is embedded in a satellite, the plurality of sources 10 thus allows separate zones of the Earth's surface to be covered simultaneously. The use of a plurality of input sources 10 in the aforementioned CTS antenna however has limits.
Firstly, the pillbox junction 23 has only a single focal point. Since the focus is perfect only for a source placed at the focal point of the reflector, defocus aberrations appear for sources 10 distant from the focal point of the reflector. These aberrations are the result of an imperfect conversion of the cylindrical waves into plane waves by the pillbox beamformer.
Moreover, as illustrated in FIG. 4, the wave emitted by an off-centred source 10 and reflected by the pillbox junction 23 in a very off-axis direction propagates obliquely in the distributor 1. To avoid reflections (single reflections or multiple reflections, from one edge to the other) of the waves from the sides of the distributor 1, it is then necessary to oversize the distributor 1 along the X-axis. This oversizing 4 of the distributor 1, which leads to an oversizing of the longitudinal radiating horns 5 along the same axis, has a cost in terms of weight, in particular in a satellite. It moreover depends on the targeted maximum pointing angle and on the propagation length in the distributor 1. It is all the larger if coverage is required over a vast angular sector along the axis of the main dimension of the longitudinal radiating horns 5, and if the electrical length of the distributor 1 is large.